The field of the invention is apparatus for proportioning and mixing liquid with powders. The materials feed into a screw conveyor-type mixing assembly designed to ensure uniformly mixed final product. Materials are metered with the delivery systems connected by a gear system so that the proper ratio of materials is maintained. The mixing apparatus of interest may be useful in food processing, making adhesives for the wood products industry and other general mixing applications requiring blending of a powder into a liquid where proportions are critical to product quality.
Multi-component adhesives are commonly used in the wood products industry in laminating, finger jointing and edge gluing. These applications will become increasingly important in the wood products industry as available trees become smaller and larger finished pieces need to be made by gluing together the small logs.
In the typical finger-jointing system, adhesive is pumped to an applicator system, incorporating a finger-type extruder head. A pneumatic control system releases glue intermittently as the work piece is advanced through the gluing machine. Finger-jointing requires small amounts of glue with many flow interruptions, as determined by a particular mill application. Glue mixing and dispensing systems for these processes are generally small in size but require constant cleaning to prevent fouling and plugging.
A typical wood jointing resin adhesive may be of the phenol-formaldehyde-resorcinol type, for example, as described by Gillern in U.S. Pat. No. 4,061,620, assigned to Weyerhaeuser Company. The resin is typically a liquid comprised of monomers and other oligomers of the basic chemical structure. These resins, when admixed with a methylene donor or hardener/curing agent, react to form an insoluble thermoset resin, useful as an exterior adhesive.
Hardeners are usually powdered paraformaldehyde or acetaldehyde, which may be blended with wood flour, clays or other fillers to help ensure rapid and uniform dispersion of the hardener in the resin.
Proper proportioning and mixing are absolutely essential in making dependable and reliable adhesives. An inadequate amount of hardener will result in poorly cured adhesives which will form weak bonds. The desired proportions of resin to hardener are those that supply a small excess of curing agent over that necessary to get total conversion to a completely infusable product, insoluble in common solvents.
Mill operations today often batch mix adhesives, particularly where only small amounts are required. This method is time consuming and sometimes results in variable and poor adhesive qualities.
Continuous machines currently available for exactly proportioning and blending powders with resins are expensive and generally not economical for the relatively small capacities needed. Many machines on the market use a weigh-belt feeder to meter powder which tends to make for a bulky, awkward machine.
Screw conveyor mixers are well known in the industry. For example, Ainsworth et al in U.S. Pat. No. 3,591,145 describes continuously mixing large amounts of dry clay with linseed oil to make putty. An extrusion stage is required to mix the "plastic" materials. The Ainsworth apparatus is designed to handle large amounts of material. There are no provisions for quick cleanout since there is little danger of the putty polymerizing in the conveyor. The Ainsworth extrusion mixer at its low turning speeds would not adequately mix the resin adhesives of interest because it would not provide the high shear rate mixing conditions necessary.
An object of the invention described below is accurate metering of powder and liquid to assure a selected ratio of ingredients. The apparatus then thoroughly mixes the components to make a finished, intimately blended product.
The mixing system is capable of mechanically dosing resin glues with a powder hardener continuously, at low capacities. The resulting adhesives are equal or better to batch-mixed materials in quality. Any combination of liquid and powder components may be mixed. For example, a glue powder plus water and liquid hardening agent could be mixed. Likewise, a liquid glue plus a powder extender and a liquid hardening agent, or a liquid glue plus a powdered hardening agent.
The simplicity of the apparatus results in capital cost savings over those described in the art. A key advantage of the system is that cleanup is facilitated. There is less waste water generated from cleanup. The mixing assembly may be removed, without tools, for washing in an appropriate solvent.